A general combustor provided in a gas turbine is provided with a pilot nozzle and main nozzles. A flame obtained by combusting fuel injected from the pilot nozzle is used as a pilot burner for combustion with the main nozzles. In the case of a dual-fuel firing combustor in which combustion is performed by selectively using fuel gas and fuel oil as fuel, there is a risk of high-temperature combustion gas flowing back to the pilot nozzle while the fuel gas is not injected as fuel during an oil-firing operation, causing burnout of the pilot nozzle. Therefore, the pilot nozzle is swept by using air guided from a casing of the gas turbine (for example, see Cited Document 1).
Furthermore, PTL 2 discloses performing staging processing, in which a predetermined number of blocks into which a plurality of main nozzles provided in the combustor are grouped are sequentially operated and stopped, thereby making it possible to prevent sudden ignition in the main nozzles when combustion with the pilot nozzle is switched to combustion with the main nozzles and to prevent damage to the gas turbine.